The city’s only public middle-school military academy may be going the way of the horseback cavalry.

Since 1998, “generals” and “colonels” at Intermediate School 286 in Harlem, also known as the Renaissance Military and Leadership Academy, have been keeping youngsters in line with military-style discipline.

The 300 or so cadets, as the students are called, wear camouflage to school, address their teachers as “sir” or “ma’am,” march in step to class, and do push-ups when they misbehave.

But cadets fear that the school’s unique tough-love approach will vanish this fall with the loss of the school’s lone commandant and military instructor, Gregory Collins.

Collins, who uses the title of colonel, will not return to Renaissance this fall because regional education officials did not renew the $54,000 contract it had with the Harlem Youth Marines, a military-style community group run by Collins.

“Without me, there’s no military presence in the school,” said Collins, who runs the Harlem Youth Marines from the Harlem Armory. “If that’s gone, then the whole military structure is gone.”

Signs that the school’s theme is fading have already surfaced. Cadets recently received a letter from the school informing them that gray slacks and black shirts will replace the camouflage uniform.

“All my friends are talking about the letter. They all think it’s a shame what’s happening to the school,” said Eric Jimmerson, 13.

As an independent contractor without a teaching license, Collins taught the only class that covered military subjects.

Principal Sandra Small, who goes by “general,” said the school would keep its name and work to retain its theme, but acknowledged that the atmosphere “definitely wouldn’t be the same.”

“He made that whole military piece,” Small said of Collins. “We’re hoping that our social studies and literacy people will be able to pick up the military [courses].”

She added that losing the camouflage fatigues was necessary because they gave the perception that the school was a “boot camp.”